Pipe-to-Soil Potential Limits for Protective Coatings.
Final rept. Nov 77-Apr 80,
HARCO CORP MEDINA OH
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This report summarizes the results of a study undertaken to determine the limiting potential criteria for cathodic protection of coated metallic underground and underwater facilities to avoid damage from hydrogen evolution. Tests of 30 days duration were performed in Houston tap water on four coatings, i.e., fusion bonded epoxy, coal tar, plastic tape and asphalt. Each coating was cathodically protected at polarized instant off potential levels of -1.02, -1.07, -1.12, -1.17, and -1.22 volts to copper-copper sulfate. It was found that hydrogen evolution is initiated at a polarized potential of -1.12, volts and becomes more vigorous as the applied current is increased. The polarized potential value increases as the current increases only up to a value of -1.22 volts. An increase in applied current beyond that value increases the hydrogen evolution and increases the ON potential, but there is no measurable increase in the OFF potential. The different coatings tested reacted differently in these tests. These short term tests should not be used for comparison of disbondment resistance. One coating experienced disbondment at an OFF potential where no gas was evident, and another coating experienced no disbondment even at -1.22 volts under vigorous hydrogen evolution. Author
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